Organic solar cells are solar cells having a structure which utilizes conjugated polymers in which double bonds are alternately arranged, such as polyparaphenylenevinylene (PPV), photosensitive low molecular weight compounds such as CuPc, perylene and pentacene, and organic semiconductor materials such as (6,6)-phenyl-C61-butyric acid methyl ester (PCBM). The organic semiconductor materials described above can be designed as desired and can be synthesized into various derivatives, so that the organic solar cells have a potential for limitless advancement.
Organic solar cells fundamentally have a thin film type structure, and mainly use a transparent electrode made of indium tin oxide (ITO) as a cathode, and a metal electrode made of aluminum (Al) or the like, which has a low work function, as a cathode. The photoactive layer has a thickness of about 100 nm and has a bulk heterojunction structure in which a hole acceptor and an electron acceptor are co-present.
As the hole acceptor, a conjugated polymer having electric conductivity, such as PPV, is used, while fullerene is used as the electron acceptor. At this time, in order to collect without loss the electrons generated by light into the aluminum electrode through fullerene, a sufficient amount of fullerene must be mixed into the conjugated polymer. Therefore, a fullerene derivative such as PCBM described above can be used so that fullerene can be well mixed with the conjugated polymer.
When the conjugated polymer absorbs light, bound electron-hole pairs (excitons) are generated, and the electrons and holes thus generated are collected at the cathode and the cathode, respectively, via fullerene and the conjugated polymer.
The organic solar cells can be mass produced at low cost with easy processability, and have an advantage that since thin film production can be achieved by a roll-to-roll system, manufacture of large-sized electronic devices having flexibility is feasible.
However, in spite of such technical and economical advantages as described above, there are difficulties in the application of the organic solar cells into practical use because of their low efficiency. Therefore, active research for an improvement of efficiency is underway in the field of organic solar cells. So far, efficiency-related research has been focused on the selection of raw materials or the design of production process for the photoactive layer for effective utilization of absorbed light, or for the electron transport layer and the hole transport layer; the shape and structure of organic thin films intended to overcome low charge mobility; an increase in crystallinity, and the like.